This invention relates in general to optical transceivers and, more particularly, to laser rangefinders.
A laser rangefinder is an optical transceiver which is capable of measuring the distance between the rangefinder and a target in the distance. The laser rangefinder includes a laser light source or laser transmitter, and further includes a photodetector or laser receiver. Typically, the laser transmitter irradiates the target with incident laser light pulses and the laser receiver receives the reflected light pulses. A signal processor in the rangefinder records the time between transmission of a pulse by the laser transmitter and reception of the reflected pulse back at the rangefinder receiver. The signal processor then uses this time difference to calculate the distance between the rangefinder and the target.
Some conventional rangefinders are capable of transmitting and receiving laser light pulses exhibiting two different wavelengths which will be referred to as first and second wavelengths. In actual battlefield conditions, the user typically selects the first wavelength which is chosen for maximum range. Although resulting in maximum range, a first wavelength, .lambda.1, of 1.06 .mu.m for example exhibits high energy content which may not be suitable for use in battlefield game exercises due to concern for visual safety. In this instance, the user selects the second wavelength, .lambda.2, for example 1.54 .mu.m, which is significantly longer and which exhibits a lower energy content than the first wavelength. Use of the second wavelength generally results in significantly reduced range in prior rangefinders.
To accommodate transmission and reception of two different wavelengths, prior rangefinders have employed two separate receive channels such as seen in the rangefinder of FIG. 1. In that rangefinder, a beam splitter is used to provide the return laser pulse to the two receive channels which include respective photodetectors. Such two channel rangefinders typically include control circuitry to switch to the appropriate channel according to which of the two wavelengths is to be detected. This results in significant switching overhead. Needless to say, such dual channel, dual wavelength laser rangefinders have a significantly higher parts count and cost than their single wavelength counterparts.
In conventional laser rangefinders, great care must be taken to mechanically and optically align the laser transmitter and the laser receiver with respect to each other. That is, optical coupling in prior laser rangefinders was accomplished through the use of discrete coupling optics, namely lenses, which necessitated tight mechanical tolerancing of receivers and transmitters to maintain boresight and field of view (FOV) requirements. Additional fixed optics were also required to ensure power densities which did not exceed the damage threshold level of the photodetector in the laser receiver of the rangefinder. Moreover, with such rangefinders there was little flexibility in the placement of the laser receiver with respect to the laser transmitter.